Phosphate is a very important nutrient for the photosynthesis of plants so that more than 90% of the global production of phosphate is processed to phosphorus fertilizers. The availability of phosphorus, however, is limited. In order to enhance the limited availability of phosphoric material there have been efforts to use sewage sludge ash, which often contains a considerable amount of phosphorus and which usually is disposed of in a landfill, as a source to recover phosphorus. It has been proposed to burn phosphorus containing sewage sludge to obtain a phosphorus containing ash with a phosphorus content of 8 to 20 wt.-%. The main chemical components of the resulting sewage sludge ashes are SiO2, CaO, Al2O3, Fe2O3 and P2O5. The ashes, however, also contain heavy metals, such as lead, cadmium, arsenic, chrome, copper, nickel, zinc or mercury As the content of these elements exceeds the limits provided by official regulations and as the plant availability of its phosphate compounds is poor, the sewage sludge ash as such cannot be used as fertilizer and has to be processed beforehand.
Application PCT/EP2012/061986 describes a process for the separation of heavy metals from phosphorus-containing sewage sludge ash, wherein the starting material is heated to a temperature of between 700° C. and 1.100° C. in a first reactor, combustion gases are withdrawn and the heated starting material is transferred to a second reactor where chlorides of alkaline and earth alkaline metals are added.
It is also known from the prior art (see Böhm, H., Werner W., “Entwicklung und Erprobung eines Verfahrens zur chemisch-thermischen Verarbeitung P-haltiger Klärschlammasche zu Düngerphosphaten”, German Federal Ministry of Research and Technology, Research Report T 81-127, July 1981) to heat a mixture of soda, phosphate-containing sewage sludge ash and sand to a temperature of between 1.100° C. and 1.250° C. in a rotary kiln. There are numerous technical disadvantages to this process: firstly heavy metals that are present in the starting material will not be removed from the resulting fertilizer; secondly the process is very energy intensive because the air within the rotary kiln, although not used in the process, needs to be heated in order to retain the required reaction temperature.